Today's semiconductor devices, whether discrete power or logic ICs, are smaller, run faster, do more and generate more heat. Some desktop microprocessors dissipate power levels in the range of 50 to 100 watts. These power levels require thermal management techniques involving large capacity heat sinks, good air flow and careful management of thermal interface resistances. A well designed thermal management program will keep operating temperatures within acceptable limits in order to optimize device performance and reliability.
Semiconductor devices are kept within their operating temperature limits by transferring junction generated waste heat to the ambient environment, such as the surrounding room air. This is best accomplished by attaching a heat sink to the semiconductor package surface, thus increasing the heat transfer between the hot case and the cooling air. A heat sink is selected to provide optimum thermal performance. Once the correct heat sink has been selected, it must be carefully joined to the semiconductor package to ensure efficient heat transfer through this newly formed thermal interface.
Thermal materials have been used to join a semiconductor package and a heat sink, and to dissipate the heat from the semiconductor devices, such as microprocessors. Thermal interface material (TIM) typically comprises a polymer matrix and a thermally conductive filler. The TIM technologies used for electronic packages encompass several classes of materials such as epoxies, greases, gels and phase change materials.
Metal filled epoxies commonly are highly conductive materials that thermally cure into highly crosslinked materials. However, they have significant integration issues with other components of the package. For example, metal filled epoxies exhibit localized phase separation within the material. This is driven by package thermo-mechanical behavior that results in high contact resistance. Furthermore, the high modulus nature of epoxies leads to severe delamination at the interfaces.
Thermal greases are in a class of materials that offers several advantages compared to other classes of materials, including good wetting and ability to conform to the interfaces, no post-dispense processing, and high bulk thermal conductivity. Greases provide excellent performance in a variety of packages; however, greases cannot be used universally with all packages due to degradation of thermal performance during temperature cycling. It is observed that in some packages greases migrate out from between the interfaces under cyclical stresses encountered during temperature cycling. This phenomenon is known as “pump out.” The extensive thermo-mechanical stresses exerted at the interface during temperature cycling are due to the relative flexing of the die and the plate of the heat sink with changes in temperature. Because the pump-out phenomenon is inherently related to the formulation chemistries utilized in greases, all typical greases are subject to pump-out.
Gels typically comprise a crosslinkable silicone polymer, such as vinyl-terminated silicone polymer, a crosslinker, and a thermally conductive filler. Gels combine the properties of both greases and crosslinked TIMs. Before cure, these materials have properties similar to grease. They have high bulk thermal conductivities, have low surface energies, and conform well to surface irregularities upon dispense and assembly, which contributes to thermal contact resistance minimization. After cure, gels are crosslinked filled polymers, and the crosslinking reaction provides cohesive strength to circumvent the pump-out issues exhibited by greases during temperature cycling. Their modulus (E′) is low enough (in the order of mega-pascal (MPa) range compared to giga-pascal (GPa) range observed for epoxies) that the material can still dissipate internal stresses and prevent interfacial delamination. Thus, the low modulus properties of these filled gels are attractive from a material integration standpoint. However, even though the modulus of the gels currently used in the industry is low, it is not low enough to survive the reliability-stressing test. The present invention provides a TIM that has lower modulus that meets the performance requirements of electronic packages and also survives the reliability-stressing test.
Phase change materials (PCMs) are in a class of materials that undergo a transition from a solid to a liquid phase with the application of heat. These materials are in a solid state at room temperature and are in a liquid state at die operating temperatures. When in the liquid state, PCMs readily conform to surfaces and provide low thermal interfacial resistance. PCMs offer ease of handling and processing due to their availability in a film form and the lack of post dispense processing. However, from a formulation point, the polymer and filler combinations that have been utilized in PCMs restrict the bulk thermal conductivities of these materials. In general pump-out, bleed-out, and dry-out are continuing reliability issues for phase change TIMs. The current olefinic PCMs are not able to provide the required reliability performance due to the higher die temperatures and increasing reliability requirements.
The description set out herein illustrates the various embodiments of the invention, and such description is not intended to be construed as limiting in any manner.